1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display apparatus for displaying a high-resolution image such as a TV picture.
2. Description of the Related Art
In recent years, liquid crystal displays have become popular in a wide variety of applications such as a display panel for computer or a word processor, measurement instrument display panel, etc. One of such liquid crystal displays is a simple matrix type in which there are provided a pair of substrates each having stripe-shaped electrodes wherein these substrates are disposed at locations opposite to each other such that the stripe-shaped electrodes of one substrate cross those of the other substrate. This type of liquid crystal display exhibits performance good enough to display clock time or a simple image. However, the time-division driving technique used in this display has a limitation in that this technique cannot drive a huge number of pixels included in a high-resolution image such as a TV picture. To avoid this problem with the simple matrix scheme, an active matrix technique has been under intense research and development in recent years.
In liquid crystal displays of the active matrix type, one substrate has a common electrode, and the other substrate has pixel electrodes corresponding to individual pixels wherein a thin film transistor (hereafter referred to as TFT) acting as a switching element is disposed at each pixel thereby controlling the driving of each pixel. Each TFT has two main electrodes called source and drain and also has a control electrode called gate. In the active matrix technique, one of main electrodes is connected to a signal line, the other is connected to a pixel electrode, and the gate is connected to a scanning line. Which of the main electrodes acts as a source electrode depends on the type of a transistor and the polarity of an applied voltage. In this description, it is assumed that an electrode connected to a display signal line acts as a source electrode, and the other electrode connected to a pixel electrode acts as a drain electrode.
FIG. 15 illustrates a circuit of a liquid crystal display of the active matrix type. In FIG. 15, reference numerals 2 and 3 denote a scanning line and signal line, respectively, and reference numeral 4 denotes a pixel electrode. Reference numeral 18 denotes a common storage capacitor electrode, and reference numeral 19 denotes a TFT. In the active matrix type liquid crystal display shown in FIG. 15, scanning lines 2 and signal lines 3 are arranged in a matrix form, wherein the operation of TFTs 19 disposed at individual pixels is controlled thereby controlling the voltage applied to the pixel electrodes 4 so that a desired image is displayed.
Top and side views of a pixel are shown in FIG. 14, wherein FIG. 14(b) is a cross-sectional view taken along line A-A' of FIG. 14(a) showing a gate electrode 1, a semiconductor layer 5, a source electrode 6, a drain electrode 7, and a light shielding layer 9.
There are also shown inter-layer insulating layers 8, a substrate 11, orientation films 12, a liquid crystal material 13, an opposite transparent electrode 14, a film 15 disposed between adjacent layers, an opposite transparent substrate 16, and a color filter 17.
As shown in FIGS. 14(a) and (b), a gate electrode 1 is formed, via an inter-layer insulating layer 8, on a semiconductor layer 5 formed on an insulating and transparent substrate 11, then an additional inter-layer insulating layer 8 is deposited on it. Contact holes are then formed in the inter-layer insulation layer 8, and source and drain electrodes 6 and 7 are formed via these contact holes. Usually, the semiconductor layer 5 is made up of a material such as polysilicon, amorphous silicon (a-Si), or single-crystal silicon. The gate electrode 1 and the scanning line 2 are made up of a material such as a polysilicon of a-Si that can be easily formed by means of evaporation. The source electrode 6 and the signal line 3 are made of metal such as Al, and the drain electrode 6 and the pixel electrode 4 are made of transparent ITO (Indium Tin Oxide). A storage capacitor electrode 18 is formed by the same process as that for the gate electrode 1, such that a storage capacitor (Cs) is formed between the storage capacitor electrode 18 and the pixel electrode 4.
A light shielding layer 9 is formed on an opposite substrate 16 located opposite to the TFT substrate wherein the light shielding layer 9 is made up of metal such as chromium, and a color filter layer is further formed on it. Furthermore, an opposite transparent electrode 14 is formed on an intermediate film 15. Usually, the opposite transparent electrode is made up of ITO.
An orientation film 12 is formed on the surface of the TFT substrate and also on the surface of the opposite substrate. In general, the orientation film is made of a material such as polyimide. After orientating process is performed on the orientation films 12, both substrates are bonded to each other via a gap element, and a liquid crystal material 13 is then placed between the substrates. Thus, a complete liquid crystal display panel is obtained.
FIG. 16 illustrates the basic concept of a conventional apparatus for driving an active matrix liquid crystal device.
This driving apparatus comprises: pixels each comprising a liquid crystal cell 701 having a liquid crystal material disposed between a pixel electrode 5 and an opposite electrode 14 (voltage V.sub.COM is applied to it), a pixel TFT 702, and a storage capacitor 712; vide signal interconnection lines (hereafter referred to as signal lines) 703; a line buffer 704; a shift pulse switch 708; a horizontal shift register 705; a gate signal interconnection (hereafter referred to as gate interconnection) 711; and a vertical shift register 706, wherein a video signal is applied via a signal input terminal and transferred to pixels or lines while varying the transfer timing for each pixel or line.
FIG. 19 illustrates the timing of driving pulses applied to the active matrix liquid crystal device according to the conventional technique. In FIG. 19, driving is performed line to line. One line of video signal V.sub.IN to be written into the liquid crystal device is supplied to the buffer 704 via the shift pulse switches 708 driven by a signal that is provided by the horizontal shift register 705 in synchronism with the frequency of the video signal, and the video signal V.sub.IN is stored in the buffer 704. When the line buffer 704 has stored the vide signal for all pixels on a certain line for example the n-th line, the pixel vide signal V.sub.LCn is written into each liquid crystal cell 701 disposed on the line via output switches 710 of the line buffer 704 wherein the output switches 710 are turned on in response to a signal S1 and via the pixel switches 702 that are turned on in response to a signal S2 provided by the vertical shift register 706. The signal transfer to each liquid crystal cell is generally performed for all cells located on a line at the same time during a blanking period in a horizontal scanning period. By varying the timing as described above, the pixel video signals V.sub.LCn, V.sub.LCn+1, . . . , are written line to line.
Liquid crystal molecules in each cell move in response to the signal voltage transferred in the above-described manner, and thus a corresponding change in the transmittance of each cell occurs depending on the orientation of polarizing plates disposed such that cross polarization holds. The change in the transmittance is illustrated in FIG. 17.
In FIG. 17, the signal voltage V.sub.SIG plotted along the horizontal axis has different meanings depending on the type of a liquid crystal used. For example, when a TN liquid crystal is used, the value of the signal voltage V.sub.SIG is defined by an effective voltage value (V.sub.rms). Referring to FIG. 18, a qualitative explanation on the value of the signal voltage V.sub.SIG will be given below. To prevent the liquid crystal from being supplied with a DC component, the polarity of the signal voltage is inverted every frame, although the liquid crystal itself operates in response to the AC voltage component represented by hatched areas in FIG. 18. Therefore, the effective voltage V.sub.rms can be written as ##EQU1## where t.sub.F denotes the two-frame time period, and V.sub.LC (t) denotes the signal voltage that is transferred to a liquid crystal.
Each TFT includes parasitic capacitance (Cgs) between its gate and source. A change in gate voltage induces a shift in the potential of the pixel electrode 4 via the parasitic capacitance Cgs. If such a shift occurs in the pixel potential, a DC voltage component can appear across the liquid crystal 13. This produces an incidental image and flicker, or causes the liquid crystal to be burned, which results in a reliability problem. The shift in the potential due to the parasitic capacitance Cgs can be suppressed by disposing a capacitor Cs at each pixel. If leakage occurs in a TFT, the pixel potential decreases and thus degradation in contrast occurs. This problem can also be suppressed by the capacitance Cs which results in an increase in the pixel capacitance and thus results in suppression of the reduction in the pixel potential.
However, since the storage capacitor electrode is generally made up of an opaque material that is also used to form the gate electrode as described above, the addition of the capacitor Cs causes a problem that the open area ratio of the pixel decreases. For example, to form a capacitor Cs having a sufficiently large capacitance such as 50 fF to obtain the above-described effect on a pixel having an area of 20 .mu.m.times.20 .mu.m, the capacitor Cs will need an area of 145 .mu.m.sup.2 if the capacitor Cs is formed using a silicon dioxide film having a thickness of 1000 .ANG.. Thus, the capacitor Cs will occupy 36% of the pixel area, which will cause a great reduction in the open area ratio. Furthermore, interconnections do not contribute to the open area. Steps at interconnections can cause disturbance in the orientation of the liquid crystal near the steps. Such a region is generally shielded by a shield layer. If this shielding is performed using the shield layer of the filter substrate, then there is an registration error of .+-. a few microns in the boding process between the filter substrate and the TFT substrate. If this registration error is taken into account, the actual overall open area ratio drops to 15-20%.
To avoid the above problem, one known technique is to form a storage capacitor suing a transparent film such as ITO. However, this technique has another unsolved problem in that reflection by the ITO film occurs which causes a reduction in the amount of light that can be used by a liquid crystal display apparatus.